Micro-Lithography
Semiconductor micro-circuits such as microprocessors, memory circuits and gate arrays are currently fabricated by the process of micro-lithography, in which microscopic patterns of conductors, semiconductors, and insulators are generated on a semiconductor wafer substrate, typically silicon. The master pattern is created, on an enlarged scale, on a photo-mask. This mask is illuminated by a very narrow band source of monochromatic light, and the pattern is optically transferred to the wafer surface via a high quality system of lenses and/or mirrors which accurately reproduce the pattern on the wafer on a reduced scale. In a well-designed optical system, the minimum feature dimension which can be resolved in the transferred pattern is determined by diffraction of the illuminating light source by the features on the mask. Traditionally, a mercury vapor discharge lamp has been used as the light source. The mercury g-line at 435 nanometers has been used in micro-lithography to achieve a pattern transfer resolution of 0.5 micrometers. More recently, micro-lithography systems have been built for the mercury i-line at 365 nanometers, allowing a resolution of 0.35 micrometers to be realized.